The present invention relates to an asynchronous transfer mode (ATM) switch for achieving a switching operation of ATM cells in a cell unit, and in particular, to a shared-buffer-type ATM switch suitable for a broadcast function.
In an ATM switch, an ATM cell (namely, a fixed-length packet) is destined to an output port based on routing information contained in a header field of the cell, thereby conducting a switching operation of the ATM cell. Consequently, the cell is delivered to the output port associated therewith. However, to achieve various operations including distribution services and conference services, it is necessary to output an identical ATM cell to a plurality of output ports. The function for the cell distribution is called a broadcast function and the ATM cell used for the function is called a broadcast cell.
Heretofore, a configuration of a shared-buffer-type ATM switch having such a broadcast function has been described, for example, in the JP-A-1-256246. According to the broadcast function of the ATM switch, a broadcast cell is temporarily stored in a broadcast cell register on an output side of a shared buffer memory such that the cell is transmitted to a plurality of output ports indicated by a broadcast routing table in a cycle of output port designation in which all output port numbers are sequentially denoted by an output counter indicating the port numbers in a periodic manner.
In the conventional shared-buffer-type ATM switch having the broadcast function, a non-broadcast cell is kept in an output wait state during a period of an output port designating cycle of output timing in which an identical broadcast cell is outputted to a plurality of output ports. Moreover, to distribute an identical broadcast cell to a plurality of output ports, there is required one output port designation cycle irrespectively of the number of output ports used for the broadcast operation. In addition, to transmitting broadcast cells, there is required a period of time which is n times that consumed for the transmission of a broadcast cell. Consequently, an output probability of non-broadcast cells is lowered due to broadcast cells, which leads to a problem of reduction in the switch throughput. According to the conventional method of outputting a broadcast cell, a check is made to decide whether or not a cell delivered from the shared buffer is a broadcast cell. If this is the case, there are produced copies of the cell to be distributed to the respective output ports. In consequence, non-broadcast cells are set to the wait state and hence there arises a problem that the memory capacity is to be increased. Furthermore, while the broadcast cell is being copied as many times as there are the predetermined output ports, non-broadcast cells are in the wait state during the copy operation, which leads to deterioration of the throughput of the ATM switch.